The present invention is in the microelectronics field and generally concerns a micro interface having improved adhesion between metal and polymeric layers which have weak intrinsic bonding characteristics. More specifically, the present invention concerns a micro interface in which isolated microscopic metal clusters are partially embedded in a polymeric layer to form a mechanically based bond upon which a continuous metal layer can be adhered by bonding to exposed portions of the isolated microscopic metal clusters.
For many years, the microelectronics industry has utilized relatively poor conductors, primarily aluminum, for metal contacts and interconnections in integrated circuits. Other metals, such as copper and silver, have superior electrical characteristics, but bond poorly with the polymeric materials used to form many microelectronic devices. Compared to aluminum, copper has superior electromigration resistance, which permits higher current loads on a sustained basis. In addition, copper has lower electrical resistance, providing many potential advantages including reduced power consumption, reduced operational temperature, smaller feature sizes, and greater levels of miniaturization. Use of copper could improve the speed and power, and reduce the size of microelectronic devices.
Notwithstanding the many known advantages of copper and similar metals compared to aluminum, aluminum has been used because of its good intrinsic bonding to commonly used microelectronic materials. Poor bonding between contact metal and polymeric material can lead to premature device failure and manufacturing difficulties. During fabrication and operation, thermal stresses generated due to thermal mismatch between the metal and polymeric layers may be sufficient to cause delamination within the structure absent sufficient adhesion between the metal and polymeric layers. As a result, metals such as copper, which form weak chemical and thermodynamic bonds with commonly used microelectronic circuit materials, have been largely avoided despite their superior electrical performance characteristics.
Particular materials to which copper and similar metals bond poorly are organic polymers, such as polyimides and other types of plastics and polyesters. Polyimides are commonly used as an interlayer dielectric in microelectronic devices, and are also commonly used for encapsulation. Such polymeric materials are favored for these applications since they are low cost, easy to handle, and offer numerous desirable properties. These properties include flexibility, good planarization, and thermal stability. The organic polymers may also be mechanically and chemically tailored to match the needs of a particular application. In addition, they admit of multiple processing methods commonly used to fabricate microelectronic devices, including plasma etching, reactive ion etching, and laser ablation.
Polyimides offer further advantages in microelectronic devices including a high glass transition temperature, permitting a wide range of fabrication temperatures, and low dielectric constants. Combined with the good thermal stability, these advantages allow the polyimides to withstand common fabrication processes, such as metal deposition, annealing, and soldering.
Aluminum and similar metals adhere well to polyimide layers because they form relatively strong chemical bonds to the atomic constituents of the polyimides, i.e. carbon, oxygen, nitrogen, and hydrogen. In contrast, copper and similar metals form weak chemical bonds with those atomic constituents. As a result, a conventional copper-polyimide interface has poor adhesion due to the limited chemical interaction across the interface.
A number of techniques have been used to increase metal-polymer adhesion. These techniques fall into three general categories: chemical, mechanical and combined. The prior adhesion enhancement techniques have, to varying degrees, achieved enhanced adhesion. However, each has some associated difficulties and disadvantages.
The typically used chemical adhesion enhancement is known as wet chemistry. Wet chemistry adds functional groups, such as carbonyls, to the polymer that can chemically react with deposited metal. One potential drawback of wet chemistry is the usual need to perform several steps: activation of the polymer surface, removal of residue, and post metalization heat treatments. The method might also result in an undesirable deep modification of the polymer surface in the micrometer range, which can adversely affect the properties of polymer thin films.
Known mechanical adhesion enhancements involve texturing of the polymer surface to increase surface roughness and surface area. Various methods are known to produce the surface roughness and surface area. Though these are referred to as mechanical enhancements because of the roughness phenomena, the actual bond is still based upon the relatively weak chemical interaction between the metal and polymer. Thus, the mechanical modification describes the polymer surface, as opposed to the actual manner of bonding. Gains in adhesion per unit area result from the increased interface surface per unit area produced by the texturing.
Prior combined adhesion enhancements include plasma etching and base treatment. In plasma etching, energetic ions, electrons, photons and highly reactive neutrals are made to interact with the polymer surface. Chemical changes result from bond breaking and preferential sputtering at the polymer surface. Reactive neutrals present in the plasma also lead to the incorporation of chemically active groups that can chemically bond with the polymer. In base treatment of polyimides, the surface of the polymer is treated with a base, such as KOH or NaOH, to convert the polyimide surface to a polyamate. Subsequent treatment with an acid transforms the polyamate surface to a polyamic acid. The polyamic surface is then partially cured to form an amorphous polyimide/polyamic acid surface which bonds better with a subsequent metal layer. Apart from complexity, these combined adhesion techniques may result in excessive etch or conversion, which then actually impedes adhesion.
Accordingly it is an object of the present invention to provide an improved metal layer to polymeric layer micro interface which overcomes the aforementioned difficulties and disadvantages.
An additional object of the present invention is to provide an improved micro interface which achieves a true mechanical interlocking between a metal and a polymeric layer.
A further object of the present invention is to provide an improved micro interface for providing mechanical adhesion between metal and polymeric layers through partially embedded discontinuous metal clusters in the polymeric layer which form a bonding base for a continuous metal layer to be formed upon.
Another object of the present invention is to provide an improved process for forming a micro interface between a metal layer and a polymeric layer including steps of depositing isolated microscopic metal clusters on a polymeric layer, heating the polymeric layer to a temperature near its glass transition temperature to cause the metal clusters to partially embed into the surface of the polymeric layer in isolated fashion, and then forming a continuous layer of metal at a lower temperature.
The micro interface of the present invention provides true mechanical bonding between a metal layer and a polymeric layer. The mechanical bonding results in a good adhesion strength between metal and polymeric layers which have poor intrinsic chemical and thermodynamic bonding, such as between copper and polyimides. According to the invention, discontinuous isolated metal clusters are mechanically trapped within the surface of the polymeric layer, with a portion of the metal clusters being exposed. The metal layer is completed by a continuous metal layer, which bonds to the exposed metal clusters. This strong bond supplements the weak intrinsic bonding between the metal and polymeric layers.
More particularly, the present interface between a polymeric layer and a metal layer includes isolated discontinuous microscopic clusters of metal partially embedded in the polymeric layer. The exposed portion of the clusters is smaller than embedded portions, so that a cross section, taken parallel to the interface, of an exposed portion of an individual cluster is smaller than a cross section, taken parallel to the interface, of an embedded portion of the individual cluster. Preferably, at least half, but not all of the height of a cluster is embedded. The metal layer is completed by a continuous layer of metal bonded to the exposed portions of the discontinuous clusters.
The micro interface of the present invention is formed by heating a polymeric layer for a specific time-temperature relationship sufficient to allow penetration of the layer by metal clusters. The discontinuous metal clusters are first deposited onto a surface of the polymeric layer while the polymeric layer is cool so that the metal clusters form in isolated fashion on the polymeric layer surface. Then the polymeric layer is heated to a temperature near its glass transition temperature for a time sufficient to partially embed the metal clusters into the polymeric layer. After cooling, a continuous metal layer is deposited upon the polymeric layer to bond with the partially embedded discontinuous metal clusters.